A partial discharge or corona has been defined by the American Society for Testing and Materials as "a type of localized discharge resulting from transient gaseous ionization in an insulation system when the voltage stress exceeds a critical value. The ionization is localized over only a portion of the distance between the electrodes of the system." For example, in the specific case of an oil-filled capacitor having polypropylene insulators and oil between parallel electrodes, partial discharge can occur in entrapped or evolved gases that displace the oil. The ionized gases of the corona are highly conductive and would result in a direct arc discharge between electrodes unless prevented from forming a complete path with an insulating barrier.
Partial discharge within the casing of electrical power apparatus, such as liquid filled power transformers and electrical reactors, are difficult to locate because the small energy content of most sources leaves no observable evidence on the surrounding insulating structure. However, while low, the energy content of partial discharges may cause a progressive deterioration of the surrounding insulation, which may lead to eventual failure of the apparatus. Therefore, it is important to detect and locate any sources of corona or partial discharge during testing of the apparatus.
Because of the difficulty in detecting and locating corona sources, many different arrangements and methods have been proposed. The usual methods fall into one of two classes, which are broadly designated electrical methods and sonic methods. Both approaches are useful, as one may be more successful in detecting and locating a certain type or location of corona source (partial discharge) than the other. For example, the electrical tests are not effective if the corona source is not in the electrical winding being monitored. Since the energy content of a corona discharge is small, sensitivity of the detecting apparatus is a problem with both general approaches, but it is more of a problem with the sonic methods. For example, the sonic methods for detecting and locating corona would be more useful if they were more sensitive, as the relatively low velocity of sound waves propagating through liquid transformer dielectric makes it attractive to locate corona sources by measuring the time for the sound wave to reach different points in the transformer, to convert this time to distance, and then to calculate the coordinates of the source. However, the accuracy of the sonic approach depends upon being able to detect the disturbances responsive to the corona discharges, and to separate the resulting signals from background noise.
One example of a prior art approach is disclosed by U.S. Pat. No. 3,801,899, Apr. 2, 1974, titled "Means for Detecting the Inception of Corona Discharges Within Induction Apparatus." The system disclosed by this patent is depicted in part by FIG. 1 attached hereto. The system is attached to an induction apparatus schematically shown as a single phase power transformer 10 comprising a high voltage winding 12, a low voltage winding 14, and an iron core 16. The transformer also comprises a grounded metal tank 18 in which the windings 12 and 14 are located, and an oil dielectric 20 within the tank. Each of the windings 12, 14 has its lower terminal connected to ground.
A high voltage terminal bushing 22 is provided for carrying current between an external phase conductor 21 and the high voltage winding 12. This terminal bushing comprises a high voltage conductor 24 and a porcelain shell 26 that supports the bushing conductor and electrically isolates it from the grounded tank 18. The bushing also includes an insulating core 28 located within the porcelain shell and surrounding the conductor 24 and oil filling the space between the core 28 and the shell 26. Between the outer shield 32 and a tubular ground sleeve 34 are two spaced-apart leads 36 and 38 connected to provide a voltage between the leads proportional to the much greater voltage between the high voltage conductor 24 and ground.
A corona-level detector and a high pass filter sense corona discharges on the high voltage power circuit phase that includes the conductors 21, 24, and the winding 12. To assist the corona detector in distinguishing between corona discharges occurring inside the induction apparatus 10 and those occurring outside the induction apparatus, the corona-level detector is enabled only during a portion of each cycle of voltage. The reason for disabling the corona-level detector is that corona in air will produce strong high-frequency components on positive crests but only weak ones on negative crests. Thus, the corona-level detector will remain off in most cases where only corona in air is present.
It is known that corona discharges produce both audible and ultrasonic pressure waves in the medium surrounding the discharge. These pressure waves generally contain a wide range of frequency components. In gases, the high frequencies are attenuated, leaving only vibrations in the audible sound range. In liquids and in some solids, the attenuation of high frequencies is not as severe, and the corona discharges will produce pressure variations rich in ultrasonic components as well as audible components. For detecting the presence of such ultrasonic components, a transducer 40 is located within the dielectric liquid 20 adjacent the grounded wall of tank 18. This transducer produces an output voltage proportional to the amplitude of the pressure wave. This output is amplified and fed to the ultrasonic pressure-wave level detector, which produces an output signal pulse if the amplitude of the signal exceeds a predetermined level. This output signal continues until the level detector is disabled at the end of a pressure-wave sampling period. The signal pulses from the two level detectors (corona level detector and ultrasonic pressure-wave level detector) are supplied to a coincidence circuit. The coincidence circuit develops an output signal if the two input signals are received during a preselected sampling period. This arrangement purportedly distinguishes between cases where radio-frequency components are being produced by corona occurring within the liquid 20 and cases where corona external to the induction apparatus 10 produces radio frequency components on negative as well as positive loops of voltage.
Another example of a prior art approach is disclosed by U.S. Pat. No. 3,707,673, Dec. 26, 1972, titled "Corona Discharge Detecting Apparatus Including Gatable Amplifiers Controlled By Flip-Flop Means." Briefly, this patent discloses apparatus for accurately detecting corona discharges occurring within the casing of transformers and electrical reactors. Mechanical disturbances within the apparatus, initiated by corona discharges, are detected by two spaced mechanical-to-electrical transducers. The signals from the two transducers are amplified and converted to unidirectional signals of like polarity. The two signals are then subtracted to cancel noise common to both signals.
The above-cited disclosures and other prior art approaches of which the present inventor is aware employ acoustic or mechanical sensors mounted on the grounded tank walls of the transformer (or other high voltage apparatus), or electrical sensors mounted on the high voltage bushings of the transformer. These approaches are useful in detecting high level discharges. However, they are not particularly useful in substations or like environments where there is a high level of outside interference. For example, such high level interference is present during impulse testing, which involves charging and discharging high voltage capacitors in the vicinity of a transformer to test the transformer's reliability in an environment with lightning. Moreover, the prior art lacks a system for detecting partial discharges through a live (high potential) tank wall. Thus, for example, in applications involving an inverted current transformer or live tank breaker, the known prior art systems cannot be used to detect partial discharge.
Another shortcoming of the prior art is that systems for locating the source of a partial discharge are unreliable. For example, systems that use triangulation and multiple acoustic sensors distributed outside the transformer tank are available, but they are susceptible to external noise and do not work well when there are multiple noise discharge sources.